Drought-Based Tipping Point for Forest Collapse with Climate Warming

The terrestrial ecosphere is a photosynthetic engine to convert sunlight into biochemical energy with biomass as the format of energy available for Earth's life communities. Climate is a major driver of this photosynthetic machine with forests playing a key role in buffering anthropogenic climate warming. Climate and biome inventory data used to differentiate between warmer and cooler areas show that the land area categorized as warmer (annual mean temperature >16oC) increased 6% since 1976 and that the warmer areas are becoming drier. A few lines of evidence indicate that tree mortality globally is accelerating due to climate warming and drought events. Theoretically, drought-driven tree mortality due to the warmer climate has a potential to drive the photosynthetic machine to a tipping point resulting in forest collapse and hence contributing to a runaway climate change. In this report, we investigate the threshold values of drought for tree mortality using tree-ring data and meteorological data that have been collected in the South-western US (SWUS). We found that when the standardized precipitation–evapotranspiration index (SPEI) approaches -1.64, trees stop growing (tree-ring width is at or near zero). We hypothesize that SPEI=-1.64 is a drought tipping point for conifer mortality in the SWUS. The predictive ability of this threshold value was evaluated using NDVI anomalies from remote sensing data for 2002 (severe drought year) in the SWUS. The average NDVI was significantly lower in areas with SPEI below the derived SPEI threshold. We are applying this analysis to the other regions with different trees.

Acknowledgement: This research was supported by PSC-CUNY CIRG- 80209-08 22.